Bacterial pathogens possess elaborate mechanisms to sense and avoid chemical hazards. One important mechanism in environmental sensing is chemotaxis, a two-component signaling pathway responsible for directional change in bacterial swimming. Recently, methyl-accepting chemotaxis proteins (MCP) fused to Heme Nitric oxide/Oxygen (H-NOX) sensor domains were identified through genomic analyses of the obligate anaerobic pathogens Clostridium botulinum and Clostridium tetani. Ubiquitous in biology, H-NOX domains are found in organisms from bacteria to mammals. A molecular understanding of ligand binding has been established. However, the signal propagation mechanisms of these domains in obligate anaerobic bacteria are still unknown. Previously characterized gas-sensing chemotaxis proteins respond indirectly to gases. For example, Aer, the oxygen sensor in E. coli, senses changes in cytosolic reduction potential as oxygen concentration changes. The H-NOX-MCP may act as a heretofore-uncharacterized direct gas sensor in which the H-NOX domain binds small concentrations of ligand and induces a panic evasion response in bacteria. In this research plan, signal output and ligand sensitivity of the H NOX-MCP protein will be investigated in the presence of oxygen or nitric oxide. To understand the mechanism of signal transmission, the molecular interactions between H-NOX and MCP domains will also be studied. Finally, in vivo chemotaxis experiments will be performed on wild type and H-NOX-MCP mutants of Clostridium sporogenes, a close relative to C. botulinum, to understand how the organism responds to steadily increasing concentrations of oxygen and nitric oxide. This investigation will expand knowledge of bacterial two-component signaling as well as characterize a novel mechanism for toxic gas sensing common to pathogenic species of Clostridium.